Conduit systems are typically used for the routing and protection of electrical conductors and the like. For example, conduits are frequently used to provide a protective path for electrical, or communications, cables within a building. Conduits may also be buried in the ground to provide connections from an existing electric utility distribution system to a particular building or building complex.
National safety codes and local building codes often dictate when a metal or plastic conduit may be used in conjunction with an electrical conductor. For example, less expensive plastic conduit is often permitted to be buried in the ground and used to bring electrical service from an underground electrical feeder to indoor power equipment. The plastic conduit, typically polyvinyl chloride (PVC), is supplied in straight lengths. A straight length of PVC conduit may be heat softened and formed into a desired bend radius, such as needed for rounding corners or for avoiding obstacles in a buried installation. The heating and forming may be accomplished at the job site using readily available conventional electric portable heaters and conduit bending jigs.
A conduit system for providing electrical service typically enters a building through a vertical penetration in the building floor. The conduit then must be continued in a vertical direction up to a wall mounted junction box, for example. The point where the conduit extends beyond the surface of the floor is an especially vulnerable point for damage to the conduit. PVC conduit may be readily damaged in such an exposed condition. Building construction is often not complete when the conduit is set in the concrete floor. Therefore, building crews may inadvertently break a PVC conduit near the floor surface level. The damage is often difficult to repair. In recognition of the vulnerability of PVC conduit if installed extending from a concrete floor, many codes do not permit PVC to be used in such an application.
A rigid metal conduit elbow positioned beneath the floor of the building and extending through the floor is an alternative to the PVC conduit elbow. The rigid metal conduit elbow is more rugged than its PVC counterpart. Unfortunately, this increased ruggedness translates into greater material, handling and installation expenses. For example, metal conduits are often formed into elbows using bending jigs. Smaller size conduits may be bent without a bending jig; however, crimping of the conduit may frequently occur requiring discarding of the crimped conduit. Larger size conduits are typically bent to a desired radius with a jig. Typically, however, one jig will not be effective for all sizes of conduit used by an installation contractor. Therefore, an expensive assortment of metal conduit bending jigs must be purchased, maintained, and transported to each job site.
Rigid metal conduits also require more expensive fittings to interconnect adjacent sections than do PVC conduits. PVC conduit sections may be readily interconnected by inexpensive plastic couplings and readily secured with a welding solvent applied to the end of the conduit and the inner surface of the coupling. In addition, a rigid metal conduit may also require expensive threading tools, the additional labor of threading the ends prior to connection to adjacent lengths, and more powerful saws to cut the rigid metal conduit.
A further disadvantage of PVC conduit is that it may readily be sawed through by the action of a pulling line as typically used for pulling an electrical conductor into the conduit. This sawing action is greatest at the elbows or offsets. Oftentimes, the elbow or offset will be encased in concrete to strengthen the PVC and minimize the damage to the conduit during installation of the cable. If the conduit is already buried beneath the earth or is in another relatively difficult location to access, repair of a damaged conduit section can be expensive and time consuming.